Assalamualaikum w.b.t. What is the ruling for kissing the hands of a person older or more knowledgeable as a sign of respect? Hope for an explanation.
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
We begin with the statement of Allah SWT:
وَاخْفِضْ لَهُمَا جَنَاحَ الذُّلِّ مِنَ الرَّحْمَةِ وَقُل رَّبِّ ارْحَمْهُمَا كَمَا رَبَّيَانِي صَغِيرًا
And lower to them the wing of humility out of mercy and say, “My Lord, have mercy upon them as they brought me up [when I was] small.” 
Originally, this statement is taken from the actions of a bird who wants to hug its chick and teach and care for it, hence it lowers its wings. It is as though Allah SWT addresses a child, “Take care of your parents by getting closer to them – in terms of your relationship with them – as what they have done for you when you were little.” Then, supplicate for them so that Allah SWT grant them love and mercy, the same way they have cared for you when you were young with their love and care and guidance or it means to love them both, for they have loved and cared for you. 
Furthermore, Islam teaches its followers to do good and treat elders with respect. This is as explained in a hadith narrated by Anas bin Malik RA, where an old man came to the Prophet PBUH and the people surrounding the Prophet PBUH are slow and hesitant in giving way for him to go towards the Prophet PBUH, the Prophet PBUH then said:
لَيْسَ مِنَّا مَنْ لَمْ يَرْحَمْ صَغِيرَنَا وَيُوَقِّرْ كَبِيرَنَا
“He is not one of us who does not have mercy on our young and does not respect our elders.” 
Al-Munawi stated that both actions are a commandment that is obligatory to be placed in their right and place. Hence, it is given to the young the right of being humble and polite, loving and caring towards them. Likewise, adults are given to them the rights of acknowledgement and respect. 
The ruling of kissing the hand of another person
Regarding this matter, al-Nawawi stated that it is sunnah to kiss the hands of the pious, knowledgeable and others who are among the people of the hereafter. Whilst, kissing the hands of another person due to the person’s wealth, status and worldly matters is makruh.  Likewise, al-Buhuti also ruled it permissible to kiss the hands and head of another person as a sign of humility, respect and honouring the other person on the condition that it is far from syahwat (desires). It is obvious that it is impermissible to do as such, for worldly purposes and from this angle, it is restricted. 
It is mentioned in al-Mausu‘ah al-Fiqhiyyah  that it is permissible to kiss the hands of a pious person, just ruler, both parents, teachers and everyone who deserves to be respected. Likewise, it is also permissible to kiss the head, forehead, and the (area) between their eyes. However, this matter is permissible if the purpose is to respect, honour, and be humble when one meets or separates from each other and far from syahwat. Furthermore, it is cited Ibn Battal, Imam Malik rejects the act of kissing the hands of another person. However, al-Abhari said that Imam Malik dislikes the act if the purpose is to praise or glorify the person, however, if the purpose is to get closer to Allah SWT due to religion, knowledge and honour, then it is permissible. 
Moreover, Muhamad bin Soleh also said that kissing the hands of another person is a sign of respect to those who are among those who are well-respected and honoured, such as one’s father, a pious person (or an elderly) and a teacher, then it is fine except if it is done then there is a concern of harm towards the person whose hands are kissed to be conceited and see himself as someone of high stature. Thus, in this situation, it is impermissible due to the mafsadah. 
The companions kissing the hand of Prophet PBUH
There are narrations that state the companion R.Anhum kissed the hand of Rasullullah PBUH. Among them is from Usamah bin Syarik, who said:
قمنا إلى النبيِّ صلّى اللهُ عليْهِ وسلَّمَ فقبَّلنا يدَه
“We then came near the Prophet (ﷺ) and kissed his hand.” 
Likewise, in a narration from Zari’ bin ‘Amir, he said:
لَمَّا قَدِمْنَا الْمَدِينَةَ فَجَعَلْنَا نَتَبَادَرُ مِنْ رَوَاحِلِنَا فَنُقَبِّلُ يَدَ النَّبِيِّ صلى الله عليه وسلم وَرِجْلَهُ
“When we came to Medina, we raced to be first to dismount and kiss the hand and foot of the Messenger of Allah (ﷺ).” 
Furthermore, there is a narration from Ibn Umar, he said:
قَبَّلْنَا يَدَ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ
“We kissed the hand of the Prophet(ﷺ).” 
Moreover, there is also a narration which states that the companions R.Anhum kissed each other’s hands. Among them is from Tamim bin Salamah, he said:
لـمَّا قَدِمَ عُمَرُ رضي الله عنه الشّامَ استَقبَلَه أبو عُبَيدَةَ بنُ الجَرّاحِ رضي الله عنه فقَبَّلَ يَدَه ثُمَّ خَلَوَا يَبكيانِ
“When Umar RA arrives in Syria, Abu ‘Ubaidah al-Jarrah RA greet him and kissed his hands. Then, both of them walked away together crying.” 
According to the above discussion, in our opinion, kissing the hands of a scholar, pious, one’s parents and others is permissible. However, the permissibility is bound on the condition that it shouldn’t be excessive until it reaches the level of glorifying and sanctifying them or there is harm such as inducing feelings of conceit in the heart of the person whose hands are kissed, desires and others. Hence, in this situation, it is impermissible.
This is based on the numerous narrations and athar of the companions R.Anhum who did as such.
May Allah SWT grant us all a clear understanding in religion. Amin.
 Surah al-Isra’: 24
 Lihat Zubdah al-Tafasir min Fath al-Qadir oleh al-Asyqar, hlm. 367.
 Riwayat al-Tirmizi (1919)
 Lihat Faidh al-Qadir Syarh al-Jami‘ al-Saghir, 5/388.
 Lihat al-Majmu‘ Syarh al-Muhazzab, 4/636.
 Lihat Kassyaf al-Qina‘, 2/157.
 Lihat al-Mausu‘ah al-Fiqhiyyah al-Kuwaitiyyah, 13/131.
 Lihat juga Fath al-Bari oleh Ibn Hajar, 11/57.
 Lihat Liqa’ al-Bab al-Maftuh, 177/30.
 Riwayat Ibn al-Muqri’ dalam al-Rukhsah fi Taqbil al-Yad (2); al-Khatib al-Baghdadi dalam al-Jami‘ li Akhlaq al-Rawi (314). Al-Qastalani menyebutkan bahawa sanadnya adalah kuat. (Lihat Irsyad al-Sari, 9/155).
 Riwayat Abu Daud (5225)
 Riwayat Ibn Majah (3704)
 Riwayat al-Baihaqi dalam al-Sunan al-Kubra (13716); Ibn Abi Dunya dalam al-Ikhwan (129)
 Riwayat al-Bukhari dalam al-Adab al-Mufrad (974)
 Riwayat Ibn al-Muqri’ dalam al-Rukhsah fi Taqbil al-Yad (13)